Vacuum processing chambers are generally used for chemical vapor depositing (CVD) of materials on substrates by supplying process gas to the vacuum chamber and applying an RF field to the gas. A number of gas distribution systems for integrated circuit processing are known, but the vast majority of known systems are designed for plasma etching or for plasma enhanced CVD (PECVD). Conventional gas distribution systems typically deliver reactants at relatively low flow rates. Showerhead gas injection and diffusive transport systems are commonly used to ensure even distribution over the substrate.
These known systems are not optimized for high density plasma CVD (HDPCVD) processes, such as encapsulation and intermetal dielectric gas filling. In HDPCVD it is important to focus the delivery of reactants such as silane related species onto a substrate, because silane and its radicals, e.g., SiH.sub.3, SiH.sub.2, SiH, and so on, have high sticking coefficients. Directing the silane preferentially onto the substrate is advantageous because it maximizes the substrate deposition rate and minimizes film deposits on various internal surfaces of the reactor.
Efficient silane utilization in HDPCVD requires the reactant gas to be directed onto the substrate from close proximity, with a high flow rate, and even distribution, to achieve high deposition rates with good uniformity and film quality. A showerhead system positioned close to the substrate is not ideal because it limits the extent of ion diffusion within the plasma which can be detrimental to plasma and deposition uniformity. Diffusive systems are not adequate for HDPCVD because they cause deposition of reactants on surfaces other than the substrate being processed. Deposition on non-substrate surfaces results in an inefficient use of the reactant gases, which necessitates higher flow rates to reach the desired deposition rate and substrate throughput. These higher flow rates are costly because of both the additional gas used and the increased pumping capacity necessary for maintaining low pressure within the processing chamber. Furthermore, deposition on non-substrate surfaces within the chamber can lead to particulate problems (flaking) caused by differential expansion between the films and chamber interior surfaces, and process shifts due to changing wall conditions. Consequently, the chamber must be cleared more often to remove these chamber deposits, which further reduce substrate throughput.
A plasma etching system has been proposed in which gas inlets supply gas into a plasma processing chamber. As shown in FIG. 1, this system includes a plasma source 110 for generating a plasma in a chamber 140 and a gas ring 167 with attached gas inlets supplying process gas into the processing chamber 140 for processing a substrate 120 on a substrate support 130. This type of system may also include an additional gas ring 160. Conventionally, the deposition rate in such a system is increased by concentrating the process gas above the substrate 120. This is typically done by changing the distance from the gas ring 167 to the substrate 120. The more the process gas is concentrated toward the area above the center of the substrate, the larger the peak deposition rate. Unfortunately, in concentrating the process gas near the center of the substrate, the deposition rate on the outer portion of the substrate may not increase as much as the center, which leads to a potential decrease in deposition uniformity.
There is thus a need for a gas distribution system which is optimized for HDPCVD and which provides both an improved deposition rate and an improved deposition uniformity.